Yea 900-1200 sounds about right. I was at my local university surplus the other day, and they had a mac pro 4,1 for sell with lower specs over all, ti was listed at $890. So I think $900-1200 is a decent amount.
Keep in mind the dual socket machines will cost you more.
They will also hold 8 sticks of RAM.
I believe 900 - 1100 sounds about right depending also on the quality of the chassis.